In recent years, with advancement in the microfabrication of semiconductor devices, the line width of circuit patterns has been reduced more and more. To meet the demand for such reduction, the wavelength of exposure light, which is used in the exposure of a resist in the lithography process, is decreased. From the generation of a half-pitch of 22 nm of pattern width, the use of exposure light including a wavelength region centered at approximately 13.5 nm, which is called extreme ultraviolet (EUV), has been studied. It is considered that by using EUV light, a reduction in pattern width, pattern size and pattern pitch not achievable in the prior art can be realized.
However, in the exposure using EUV light, it is necessary to use a reflective exposure mask and optical system, instead of a transmissive exposure mask and optical system in the prior art. The reason for this is that in the case where the exposure mask is formed with a desired thickness, there is no mask material at present, which can pass EUV light of approximate wavelength 13.5 nm.
In the case of using a reflective mask, if radiation of EUV light to the reflective mask is continued, C-based contamination is accumulated on the reflective mask, leading to problems arising from such contamination. (See, e.g., Y. Nishiyama, et al., “Carbon contamination of EUV mask: film characterization, impact on lithographic performance, and cleaning”, Proc. SPIE 6921, 692116-1 [2007]). For example, if such contamination is accumulated on the reflective mask, the reflectance of the reflective surface of the reflective mask decreases, and also the mask pattern dimensions vary. As a result, wafer transfer pattern dimensions vary, the process margin decreases and the yield decreases. In addition, owing to a difference in the state of accumulation on the reflective mask, there occurs a difference in optimal exposure at a time of wafer transfer. It is necessary, therefore, to monitor the accumulation of contamination on the reflective mask.
Methods of monitoring the contamination are generally classified into a method of directly measuring the contamination on the reflective mask and a method of monitoring the reflective mask, based on the variation of wafer transfer pattern dimensions. In the case of directly measuring the reflective mask, the reflective mask needs to be moved from an exposing device to a mask measuring device, and there arise such problems as a down-time of the exposing device due to the movement or the adhesion of particles. On the other hand, there are no such risks in the method of monitoring the reflective mask, based on the variation in wafer transfer pattern dimensions. In this case, however, not only the variation in dimensions due to the contamination accumulated on the reflective mask, but also the variation in dimensions due to contamination, etc., accumulated on, e.g., a mirror within the exposing device (optical system) is measured. As a result, the contamination accumulated on the reflective mask and the contamination accumulated on the mirror within the exposing device cannot be distinguished. Hence, there arises such a problem that the determination of the contamination accumulated on the reflective mask is very time-consuming.